1. Field of the Invention
Embodiments of the invention generally relate to methods for processing a substrate, and more particularly, to methods for measuring a dopant concentration of a substrate during a doping process.
2. Description of the Related Art
It is important to control ion dosage during plasma processes, such as plasma-enhanced chemical vapor deposition (PE-CVD) process, high density plasma chemical vapor deposition (HDP-CVD) process, plasma immersion ion implantation process (P3I), and plasma etch process. Ion implantation processes in integrated circuit fabrication particularly require instrumentation and control to achieve a desired ion dose on a semiconductor substrate.
The dose in ion implantation generally refers to the total number of ions per unit area passing through a surface plane of the substrate being processing. The implanted ions distribute themselves throughout the volume of the substrate. The principal variation in implanted ion density (number of ions per unit volume) occurs along the direction of the ion flux, usually the perpendicular (vertical) direction relative to the substrate surface. The distribution of ion density (ions per unit volume) along the vertical direction is referred to as the ion implantation depth profile. Instrumentation and control systems for regulating ion implant dose (ions per unit area) are sometimes referred to as dosimetry.
Ion implantation may be performed in ion beam implant apparatus and in plasma immersion ion implantation apparatus. Ion beam implant apparatus, which generate a narrow ion beam that must be raster-scanned over the surface of the substrate, typically implant only a single atomic species at one time. The ion current in such an apparatus is precisely measured and integrated over time to compute the actual dose. Because the entire ion beam impacts the substrate and because the atomic species in the beam is known, the ion implant dose can be accurately determined. This is critical in an ion beam implant apparatus, because it employs a DC ion source, which is subject to significant drift in its output current, and the various grids and electrodes employed in the beam implant machine drift as well (due to the susceptibility of a DC source to accumulation of deposited material on component surfaces). Accordingly, precise dosimetry is essential in an ion beam implant apparatus. The precisely monitored ion beam current is integrated over time to compute an instantaneous current implant dose, and the process is halted as soon as the dose reaches a predetermined target value.
In contrast, plasma immersion ion implantation reactors present a difficult problem in dosimetry. Typically, the atomic weight of the ions incident on the substrate cannot be precisely determined because such a reactor employs a precursor gas containing the desired ion implantation species as well as other species. For example, plasma immersion ion implantation of boron usually employs a multi-element compound, such as the precursor diborane, so that both boron and hydrogen ions may be incident on the substrate. As a result, determining the boron dose from a measured current is difficult. Another difficulty in implementing dosimetry in a plasma immersion ion implantation reactor is that the plasma ions impact the entire substrate continuously, so that it is difficult to effect a direct measurement above the substrate of the total ion current to the substrate. Instead, the dose must be indirectly inferred from measurements taken over a very small area. This is particularly true of reactors employing RF (radio frequency) plasma source power or RF plasma bias power.
Therefore, there is a need for a method for determining an end point at a predetermined dopant concentration during a plasma doping process.